<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>

  <body>
    <button>点击</button>
    <script>
      const btn = document.querySelector("button");
      // 事件源.on事件=function() ,后面覆盖前面

      btn.onclick = function () {
        alert(6666);
      };
      btn.onclick = function () {
        alert(777);
      };
      事件源.addEventListenser(事件, 回调函数);
      btn.addEventListener("click", function () {
        alert(777);
      });
      btn.addEventListener("click", function () {
        alert(888);
      });
    </script>
  </body>
</html>
